Copper element or copper-containing metal oxides are widely used in the industry, and in particular, many examples of their use are found in the co-catalyst and antibacterial fields. On use as the co-catalyst, they are currently used for the sensitization of photocatalysts to visible light, and as asymmetric catalysts for organic synthesis, ATRP polymerization catalysts for polymers, catalysts for steam modification of various organic compounds. It is also known from the past that copper element or copper ions have antibacterial activity. Even in recent years, research works are made on the mechanism of achieving bacteriostasis to sterilization via enzyme inhibition of target bacterium, denaturation of membrane protein, or denaturation of constitutional protein of organelle. While copper-containing catalysts are manufactured in expectation of these effects, they are generally manufactured by adding metal copper ions or as a single metal oxide having microparticulate copper oxide supported thereon. However, since these copper-containing metal oxides change their copper composition in the ambient environment including light, heat and moisture, all oxides are expected to exert high activity in catalyst sensitization and bacteria control at the initial, but deteriorate and lose their activity within several weeks at the fastest, raising a problem in the application where their activity must be sustained over a long term.
Titanium oxide finds use in a variety of applications, for example, pigments, UV screeners, catalysts, photocatalysts, catalyst carriers, adsorbents, ion exchangers, fillers, reinforcements, ceramic stock materials, precursors to complex oxides such as perovskite type complex oxides, and primers for magnetic tape.
Among others, photocatalytic titanium oxide fine powder is a material having photocatalytic activity under exposure to light including UV light of 400 nm or shorter. The photocatalytic activity refers to the action that positive holes and electrons generate by excitation of UV light of 400 nm or shorter, diffuse to a surface, and act with molecules adsorbed to the surface to undergo redox reaction. By the redox reaction, organic matter is decomposed when adsorbed to the titanium oxide surface. Once titanium oxide fine particles having such photocatalytic activity are applied to the substrate surface to form a photocatalyst thin film, harmful organic materials adsorbed thereto can be decomposed by irradiating excitation light. They are often used in applications including cleaning of the substrate surface, deodorizing, and sterilization. It is required to increase the contact area of photocatalyst particles with a material to be decomposed for the purpose of enhancing photocatalytic activity, and film transparency is required for the purpose of preserving the aesthetic appearance of a substrate to be coated. To meet these requirements, titanium oxide fine particles in the titanium oxide dispersion must have an average particle size of not more than 50 nm.
Furthermore, titanium oxide exerts satisfactory photocatalytic activity under exposure to light in the UV region of relatively short wavelength (wavelength 10 to 400 nm) in sunlight or the like, but is sometimes awkward to exert satisfactory photocatalytic activity in an indoor space with lighting from a light source mainly producing light in the visible region (wavelength 400 to 800 nm) such as fluorescent lamps. Recently, a tungsten oxide photocatalyst (Patent Document 1: JP-A 2009-148700) attracts attention as the visible-light-responsive photocatalyst. Since tungsten is a rare element, it is desired to enhance the visible light activity of a photocatalyst using a common element, titanium.
The general processes for preparing titanium oxide fine particles, which are industrially implemented, include the sulfate process using ilmenite ore or rutile ore and the chloride process (Non-Patent Document 1: Titanium Oxide, Gihodo Publishing Co.), as well as the hydrolysis/firing process, reaction in organic solvents, and the solid phase process (Non-Patent Document 2: Standard Research of Photocatalysts, Tokyo Tosho Co.). The resulting titanium oxide fine particles are subjected to ultrafine dispersion treatment into a coating liquid in order that titanium oxide fine particles be coated onto the substrate surface while maintaining the aesthetic appearance of the substrate to be coated. The general fine dispersion treatments include, for example, methods of dispersing a synthesized titanium oxide fine powder into dispersing medium with the aid of dispersing agents such as organic dispersants by a wet dispersing machine (Patent Document 2: JP-A H01-003020, Patent Document 3: JP-A H06-279725, Patent Document 4: JP-A H07-247119, Patent Document 5: JP-A 2004-182558), and methods of stably dispersing titanium oxide into dispersing medium by surface treatment of the titanium oxide (Patent Document 6: JP-A 2005-170687, Patent Document 7: JP-A 2009-179497). However, these manufacturing methods have the problem that since ultrafine particles with an average particle size of not more than 50 nm are prone to agglomerate, a great deal of labor is necessary in order to achieve dispersion up to primary particles, and in some cases, it is impossible to disperse up to primary particles; and the problem that when particles are surface treated with inorganic or organic components or when dispersing agents such as surfactants are added, both for the purpose of enhancing dispersion stability, the photocatalyst is surface-coated therewith, which becomes a factor of inhibiting the photocatalyst from exerting activity.
Also disclosed are the method of preparing an anatase type titanium oxide dispersion having long-term stability by hydrothermal treatment of a peroxotitanic acid solution obtained by dissolving titanium hydroxide with the aid of hydrogen peroxide (Patent Document 8: JP-A H10-67516); the method of preparing rutile type titanium oxide sol (Patent Document 9: JP-A H02-255532); and the method of preparing titanium oxide sol (Patent Document 10: JP-A H10-182152). In these dispersions, titanium oxide fine particles are kept dispersed without a need for surface treatment or dispersing agents while the average particle size is not more than 50 nm. Photocatalytic coating films obtained by coating substrates with the dispersions exhibit high transparency and activity under UV light exposure, but not sufficient visible light activity.